You use a clean new disk? Don't you use the disk removed from other products? 
Somehow picture shows the name of LS-WX.(2 drives model...) 
BTW, please try this.
1) Remove the new disk and shut down the NAS.
2) wipe the new disk to create clean disk. (no partition, no data, no foramtted) 
3) pull it back the NAS.
4) reboot it.